Application Guide
Low pressure lift ejector system
Multi Ejector solution incl. Multi Ejector Low
Pressure (LP 935/1435/1935) and AK-PC 782A
Application Guide | Low pressure lift ejector system
Contents |
1. General description........................................................................................................................................................... |
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1.1 |
System design with Low Pressure lift ejector.............................................................................................. |
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1.2 |
Example system load............................................................................................................................................ |
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MT (Medium Temperature) evaporators load 50 kW................................................................................. |
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LT (Low Temperature) evaporators load 10 kW........................................................................................... |
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1.3 |
Overall control strategy and objectives......................................................................................................... |
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2. Configuration PC782A..................................................................................................................................................... |
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2.1 |
Select plant type.................................................................................................................................................... |
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2.2 |
Suction group MT (Capacity control).............................................................................................................. |
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2.3 |
HP control................................................................................................................................................................. |
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2.4 |
Condenser fan control.......................................................................................................................................... |
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2.5 |
Receiver control...................................................................................................................................................... |
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2.6 |
IO configuration..................................................................................................................................................... |
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3. What is an ejector, and how does it work?.......................................................................................................... |
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3.1 |
Danfoss Multi Ejector design.......................................................................................................................... |
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3.2 |
Multi Ejector Capacity Control....................................................................................................................... |
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3.3 |
How does the Multi Ejector solution work?............................................................................................... |
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4. Coolselector®2 Selecting components.................................................................................................................. |
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4.1 |
LP ejector selection............................................................................................................................................ |
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4.2 |
GBV, Gas By-Pass #1 valve selection............................................................................................................. |
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4.3 |
Check valve selection........................................................................................................................................ |
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4.4 |
Evaporator EEV selection.................................................................................................................................. |
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5. Multi Ejector Solution™................................................................................................................................................ |
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2 | AB322920563002en-000103 |
© Danfoss | DCS (vt) | 2020.08 |
Application Guide | Low pressure lift ejector system
1. General description |
Low pressure lift ejector systems are simpler |
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systems than high-pressure ejector systems, |
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because they do not need the parallel |
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compressor in order to work and give an energy |
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saving (see diagram on next page). In winter |
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time, the system works like a normal booster |
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system where the gas bypass valve controls the |
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receiver pressure with set pressure differential, |
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and the evaporation pressure is controlled by |
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adjusting the compressor capacity according to |
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the needed MT evaporators’ suction pressure. |
At 23 – 25 °C out of the gas cooler, the ejector starts to pump all gas from the MT evaporators and lift to the receiver with a pressure lift of approximately 3 bar. The consequence of this is a better COP of the system. The higher suction pressure will also result in higher mass flow in the same compressor and thereby a first cost saving due to a smaller installed capacity. The first cost saving can be as much as 30% at high ambient temperatures and 15% energy saving compared to a booster system.
1.1System design with Low Pressure lift ejector
The LP ejector can be considered as an add-on to the standard system, however there are some additional components, and a different control strategy needs to be applied.
Below are the most important points to take into account when designing such a system (see diagrams on page 4):
Setpoints and operational modes
As described, an LP ejector system can operate with two different modes: the standard mode in low ambient temperatures and the ejector mode in high temperatures. How it changes from one mode to the other and the setpoint and signals required for this are listed below:
•In the winter condition, the ejector is working as a high-pressure valve and is not providing any or not enough suction mass flow to put the system into ejector operation. The system is performing as a standard system, but with receiver pressure controlled with a pressure differential of 3 bar between the pressure
in the receiver (P-rec) and the common MT evaporators’ suction line. The pressure signal is coming from the sensor located before the check valve (Po-MT). The pressure in the gas cooler is controlled based on the refrigerant
outlet temperature (Sgc) and the optimum COP algorithm.
•Ambient temperatures of 17 – 18 °C result in 24 – 25 °C out of the gas cooler measured by the temperature sensor Sgc, corresponding to gas cooler pressure 64 to 66 bar(a). At ambient temperatures of 17 – 18 °C there is enough expansion work in the system for the LP ejector to take all the mass flow from the MT evaporators and lift it to the receiver. Since the gas bypass valve is controlled on the basis of pressure difference between the receiver and MT evaporator pressure, and not fixed pressure, the valve will start to open more and more. Setpoint for the valve would typically be 3 bar difference and if the ejector can provide a higher lift, the gas bypass valve will be 100% open.
Components
The system requires some additional components. Additionally the design criteria for some components are different:
•Gas Bypass Valves: It will be challenging to have one valve fulfilling good control in
both Standard and Ejector mode. In the first mode there is limited amount of gas released to suction MT compressors with a constant pressure difference of 3 bar, while in the second
one (LP ejector mode) there is a large amount of gas with a minimum possible pressure drop; our solution is with 2 x Gas Bypass Valves (GBV). GBV #1 is a standard one and works in all conditions, and GBV #2 mounted in parallel with big KV value operating when the system turns into LP Ejector mode. The idea is to have
as little as possible pressure drop across the two GBVs, as this pressure drop will count as loss to the LP ejector’s recovered work.
•Another option is to use a 2-way ball valve motor mounted in parallel to GBV #1 which can give even better system performance as the pressure drop in the gas bypass line will be minimal.
•Pressure sensors: The reference sensor for controlling MT compressors should be mounted on the common MT evaporators’ suction line “Po_MT”, before the check valve. In systems with MT and LT evaporators it can happen
that the MT evaporator load drops below the minimum MT compressor capacity which will lead to pump down situation and possible turning off of the MT compressors. On the other hand, LT evaporators can have a load and LT compressors will run. To protect LT compressors’ from high pressure cut-out, a new Psuc_MT sensor is introduced. If the pressure exceeds the set offset pressure, the Psuc_MT will become reference sensor for controlling the MT compressors.
•Electronic Expansion Valves and evaporators: The LP ejector pressure lift is relatively low. Therefore, the system needs to be designed accordingly, and the EEV (Electronic Expansion Valve) for MT evaporators should be selected according to the system requirements and limitation. The pressure losses in evaporator and distributors should be investigated and designed to be a portion of the minimum pressure lift that the ejector can provide (3 bar). Larger evaporators and distributors
may provide too high pressure loss for the LP system. If the losses in the evaporator are high, then it should be ensured that the system
will operate with high pressure in the gas cooler to provide adequate pressure lift. That can either occur when there is high ambient temperature or when Heat Recovery is activated in the system. The pressure losses should be considered in the selection of the EEV
•Compressors: Compressor selection is made using the receiver pressure at the highest ambient temperature (design condition). This is
© Danfoss | DCS (vt) | 2020.08 |
AB322920563002en-000103 | 3 |
Application Guide | Low pressure lift ejector system
typically 6 – 7.5 bar higher than the evaporation pressure and will yield smaller compressors typically 20 – 30%, resulting in reduction of the first cost of the system. The small compressor steps in the system can be useful in the part load situation during the cold periods.
Oil management
In booster/winter mode, the system has a safe oil return, but when the system is in Ejector mode, the oil will end in the receiver and will stay there if no action is taken on it. There are many ways to recover the oil and get it back to the compressor. If the oil separator has a low efficiency, the oil problem will be bigger. Part of a safe oil return can be the LT compressors. The LT compressors receive an oil-rich gas mixture from
the LT evaporators. The oil is passed through the LT compressors and can be transferred to the MT compressors as LT discharge is connected directly to the MT compressor suction. However, as the LT evaporators’ load varies during the year, sufficient oil return cannot be expected. That depends on the load ratio between the MT and LT compressor capacity, the oil quantity in the system and the efficiency of the oil separator.
To be certain, an oil recovery system is needed. By using the second pressure differential reference “DeltaP High” and a low oil level sensor mounted on the oil receiver, the system can
be changed from LP Ejector mode to Standard mode. This ensures that oil will return to the MT compressors.
1.2 Example system load MT (Medium Temperature) evaporators load 50 kW LT (Low Temperature) evaporators load 10 kW
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150.0 |
250.0 |
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Pressure in receiver controlled by pressure differential. |
Enthalpy kJ/kg |
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Parameter DeltaP low = 3 bar |
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Pressure in receiver result of LP ejector lift > 3 bar
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Enthalpy kJ/kg
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© Danfoss | DCS (vt) | 2020.08 |
Application Guide | Low pressure lift ejector system
1.3Overall control strategy and objectives
The overall control objective is to maintain a sufficiently low pressure difference between the receiver pressure (Prec) and the evaporation
pressure (Po-MT) in order for the LP-ejector to lift the total refrigerant flow from the refrigerated cabinets up the receiver pressure level. The designated MT-compressor will in this case compress the gas (vapor) refrigerant directly from the receiver. The pressure difference between the sensors Prec and Po-MT, should however be large enough for the injection valves to supply the required refrigerant flow to the refrigerated cabinets. If the motive energy for the ejector is not big enough to perform the required pressure lift, the check valve will open and the MT
compressor will take the refrigerant flow directly from the refrigerated cabinets (instead of the ejector).
Note that in LP ejector mode, the check valve restricts the flow direction from the compressor suction port (Psuc-MT) towards the MT evaporator outlet (Po-MT). This means that if the suction pressure (Psuc-MT) increases, it cannot be detected in the Po-MT measurement. It is therefore important to device a control strategy that keeps Psuc_MT in control as well.
In the section below the control strategy and configuration of the PC782A to support the LPejector application will be described.
2.Configuration PC782A In the PC782A, the LP-system is not considered as
a dedicated “Plant type” but rather a special case of a booster system (without IT compressor). This means that in order to support the LP-ejector system, a number of configurations need to be made in respectively: ”Select Plant Type”, “Suction Group MT”, “HP-control”, “Receiver control” and “I/O configuration”.
2.1 Select plant type |
The LP-ejector system is considered a Booster/ |
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One pack system without any IT compressors, |
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therefore it is only possible to configure the Delta |
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P control of the receiver from these plant type |
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selections: "One Pack + HP" and "Booster + HP". |
© Danfoss | DCS (vt) | 2020.08 |
AB322920563002en-000103 | 5 |
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